This invention relates to broadband optical retardation devices, and is concerned more particularly with the use in such devices of patterned optical retarders in which the optic axis varies over the retarder in accordance with a predefined pattern.
By a xe2x80x9cbroadband optical retardation devicexe2x80x9d is meant a retardation device for light consisting of a broad range (of the order of tens or hundreds of nanometers) of wavelengths which constitutes at least a part of the complete wavelength range of visible light and ultraviolet and infrared radiation.
Such optical retardation devices may be used for polarisation encoding of display information or in diffractive optical systems, for example.
S. Pancharatnam xe2x80x9cAchromatic Combination of Birefringent Platesxe2x80x9d, Proceedings of Indian Academy of Sciences vol XLI, No 4, Sec A, 1955, pp 130-136 and pp 137-144, discuss the use of stacked uniform retarder films to improve achromaticity, that is the independence of its light transmission properties from the wavelength of the input light. Use is made of combinations of uniform retarder films having different azimuthal orientations of their optic axes. A method for calculating the required retardations and orientations of the optic axes is also given. However these references are concerned only with combinations of uniform retarders, that is retarders whose optic axis is substantially invariant over the retarder.
It is known to fabricate patterned retarders, that is retarders whose optic axis vary over the retarder, for example between first and second regions which alternate in x and/or y directions over the plane of a layer. European Published Patent Application No. 0689084 (Schadt) proposes the use of reactive mesogen layers as optical elements and alignment surfaces.
It is an object of the invention to provide an optical retardation device having improved achromaticity which is capable of generating broad band orthogonal optical modes.
According to the present invention there is provided a broadband optical retardation device for receiving light consisting of a broad range of wavelengths, the device including patterned optical retardation means including a first region having a first optic axis at an orientation a between 0xc2x0 and +90xc2x0 to a reference plane and a second region having a second optic axis at an orientation b between 0xc2x0 and xe2x88x9290xc2x0 to the reference plane, and non-patterned optical retardation means having an optic axis at a defined orientation c, greater than the orientation a and less than the orientation 180xc2x0+b, to the reference plane in order to increase the achromaticity of the light polarised by the combination of the patterned optical retardation means and the non-patterned optical retardation means as compared with the light polarised by the patterned optical retardation means alone.
It should be understood that, in the above definition and elsewhere in the specification, the term xe2x80x9coptic axisxe2x80x9d is used to denote the so-called slow optic axis of the material referred to.
Such a combination of patterned optical retardation means and non-patterned optical retardation means in this manner enables generation of broad band orthogonal optical modes in a straight forward manner, and permits a broad bandwidth response to be obtained with improvement in the quality and/or ease of fabrication as compared with known broad band optical retardation devices.
The first and second regions of the patterned optical retardation means are preferably such as to polarise input light linearly polarised along the reference plane such that, after passing through the non-patterned optical retardation means, the light which has passed through the first region is orthogonal to the light which has passed through the second region. It will be understood that the required orthogonal relationship between the polarised light from the first regions and the polarised light from the second regions can be satisfied whether the light is linearly polarised or circularly polarised.
In a preferred embodiment of the invention the orientations a and b of the first and second axes of the patterned optical retardation means are substantially equal and opposite relative to the reference plane, and the orientation c of the optic axis of the non-patterned optical retardation means is substantially perpendicular to the reference plane. Such an arrangement optimises the broad band response of the device.
The orientations a and b of the first and second axes of the patterned optical retardation means are preferably in the ranges of +10xc2x0 to +75xc2x0 and xe2x88x9210xc2x0 to xe2x88x9275xc2x0 respectively, and most preferably in the ranges of +10xc2x0 to +35xc2x0 and xe2x88x9210xc2x0 to xe2x88x9235xc2x0, relative to the reference plane. The optimum response is obtained if the first and second orientations a and b of the first and second axis of the patterned optical retardation means are about +22.5xc2x0 and xe2x88x9222.5xc2x0 respectively relative to the reference plane.
In one embodiment of the invention the orientations a and b of the first and second optic axes of the patterned optical retardation means and the orientation c of the optic axis of the non-patterned optical retardation means substantially satisfy the relationships c=a+45xc2x0 and b=cxe2x88x9245xc2x0.
The patterned optical retardation means may include a patterned uniform layer having an optic axis which varies between the first and second regions along one or more directions x and y parallel to the layer, but which does not vary substantially through the thickness of the layer.
Alternatively the patterned optical retardation means may include a patterned twisted retardation layer having an optic axis which varies between the first and second regions along one or more directions x and y parallel to the layer and also through the thickness of the layer.
Furthermore the non-patterned optical retardation means may include a uniform retardation layer whose optic axis has an orientation which does not vary substantially through the thickness of the layer.
Alternatively the non-patterned optical retardation means may include a twisted retardation layer whose optic axis has an orientation which varies through the thickness of the layer.
The invention also provides an optical retardation device including patterned optical retardation means including a first region having a first optic axis configuration and a second region having a second optic axis configuration, and non-patterned optical retardation means having a further optic axis configuration, at least one of the optic axis configurations being a twisted optic axis configuration whose optic axis has an orientation which varies through the thickness of a layer, whereby the achromaticity of the light polarised by the combination of the patterned optical retardation means and the non-patterned optical retardation means is increased as compared with light polarised by the patterned optical retardation means alone.
In one embodiment of the invention the first optic axis configuration has an average optic axis orientation a between 0xc2x0 and +90xc2x0 to a reference plane, the second optic axis configuration has an average optic axis orientation b between 0xc2x0 and xe2x88x9290xc2x0 to the reference plane, and the further optic axis configuration has an average optic axis orientation c, greater than the orientation a and less than the orientation 180xc2x0+b, to the reference plane.
In an alternative embodiment of the invention the first optic axis configuration has an output director orientation a between 0xc2x0 and +90xc2x0 to a reference plane, the second optic axis configuration has an output director orientation b between 0xc2x0 and xe2x88x9290xc2x0 to the reference plane, and the further optic axis configuration has an output director orientation c, greater than the orientation a and less than the orientation 180xc2x0+b, to the reference plane.